The process by which chromosomal replication is accurately completed is not well understood. To maintain genomic integrity during replication, cells must have an ability to recognize replicated regions, limit ongoing replication forks, and join converging daughter strands without gaining or losing a nucleotide. This process is likely to be a fundamental step in regulating accurate genomic duplication in all cells and must occur with remarkable efficiency. Failure to complete replication precisely would have disastrous consequences for the cell, producing genomic regions that are overreplicated or underreplicated, and leading to a loss in genomic stability, which is a primary factor in the cellular progression of most heritable and sporadic forms of cancer. Mutations in E. coli recBC and recD have been shown to inactivate the cell's ability to complete replication on plasmids and bacteriophage, and some evidence suggests that they may function similarly on the chromosome as well. The goal of this research project is to characterize the final steps of chromosomal replication and to define the roles of recBC and recD in this process. These experiments are necessary to characterize this critical, yet heretofore unexamined, mechanism for accurate chromosomal replication.